Conventionally, there has been known a dimming device that controls a dimming level of a luminaire by using a time-division multiplexing transmission system (see, e.g., Japanese Unexamined Patent Application Publication No. 2000-12241). A dimming device disclosed in the above reference applies a dimming signal to a dimming signal line in the form of DC voltage ranging from 0 V to 10 V. The dimming level of the luminaire becomes the lowest when a dimming signal is 0 V, and the dimming level of the luminaire becomes the highest when a dimming signal is 10 V.
Recently, LED luminaires are widely being used. Such LED luminaires are able to emit light at lower illuminance than the fluorescent lamps. At low illuminance, however, there occur differences in brightness of lights emitted from respective LED elements. For this reason, when an LED luminaire is dimmed to a low illuminance by the dimming device disclosed in the above reference, the visual quality may become poor due to unevenness in brightness or flickering. In addition, when an LED luminaire is lighted at a high illuminance by the dimming device disclosed in the above reference, there may be dazzling or glaring due to the light-emitting characteristic of LEDs.